Ceramics materials are used as various structural materials, because they are light in weight and high in rigidity, as compared to metal materials such as iron. In particular, alumina (Al2O3) and silicon carbide (SiC) are known as ceramics materials having a relatively high rigidity.
Alumina is a relatively low-cost ceramics material and thereby widely used. As for material properties, it has: a theoretical density of about 4.0 g/cm3; a Young's modulus of about 390 GPa, in the form of a dense sintered body; and a specific rigidity (Young's modulus/specific gravity) of about 100 GPa. Silicon carbide has a theoretical density of about 3.2 g/cm3. That is, Silicon carbide has a specific gravity which is less than that of alumina and relatively low among commonly-used ceramics materials. Silicon carbide exhibits high rigidity, specifically, a Young's modulus of 400 GPa or more, in the form of a dense sintered body, and a high specific rigidity of about 130 GPa.
However, for example, in a mechanical part such as a stage to be moved at high speeds, there is a need for a higher-rigidity material or a lighter-weight material even with the same level of rigidity, in order to achieve higher-speed and higher-accuracy drive control.
Boron carbide is a ceramics material having the third-highest hardness, after diamond and cubic boron nitride, and used for an abrasion-resistant part and a polishing abrasive. Boron carbide is a ceramics material having a relatively low specific gravity, specifically, a density of about 2.5 g/cm3. In addition, boron carbide has a high covalent bonding property, so that it can achieve a high-rigidity ceramics material having a Young's modulus of about 400 GPa and a specific rigidity of 150 GPa or more.
On the other hand, it is known that boron carbide is thermally stable due to the high covalent bonding property, and thereby extremely low in sinterability. For this reason, generally, a pressure sintering process such as a hot press (HP) process is employed in production of a boron carbide sintered body. However, the pressure sintering process such as a hot press process has difficulty in producing an article having a complicated shape, and gives rise to an increase in production cost. Therefore, there is a need for a technique of producing a dense boron carbide sintered body by a conventional pressureless sintering process.
In order to achieve densification of boron carbide by the pressureless sintering process, it is necessary to use a sintering aid. As a sintering aid for boron carbide, there has been known a type adding carbon, as disclosed, for example, in the following Patent Document 1. This technique makes it possible to obtain a boron carbide sintered body having a relatively high density. However, when carbon is used as a sintering aid, sintering will progress through solid-phase diffusion. In this case, it is necessary to be sinter at a high temperature, for example, of 2300° C., to densify boron carbide. A furnace usable for this high-temperature sintering is limited to a specific type, because such a high temperature is equal to or greater than maximum operating temperatures of commonly-used carbon furnaces. Moreover, the high-temperature sintering causes significant wear damage to a heater, a thermal insulator and other components of the furnace, and an increase in energy cost, which leads to an increase in production cost. Therefore, there is a need for a technique of sintering boron carbide at a lower temperature.
A technique using another type of sintering aid such as metal boron, metal silicon or carbon powder is disclosed, for example, in the following Patent Document 2. However, the technique requires performing a heat treatment in a vacuum at a temperature of 1600 to 2100° C. and then performing sintering for densification, in an inert gas atmosphere. That is, it is necessary to perform the heat treatment and the sintering in a two-stage manner, or change a sintering atmosphere from a vacuum to an inert gas atmosphere during heating, which causes a problem that a production process becomes cumbersome and complicated.
The following Patent Document 3 discloses a technique using, as a sintering aid, at least one of an Al element-containing substance and a Si element-containing substance. In specific examples, Al+Si, Al4C3+Si, or Al4SiC4, is used as the sintering aid. This technique makes it possible to obtain a dense boron carbide-based ceramics material by a pressureless sintering method at a relatively low temperature. However, boron carbide is a ceramics material having a relatively high hardness, and thereby having extremely low processability (machinability, etc.). It is known that the hardness of a ceramics material becomes lower along with an increase in grain size as long as the grain size falls within about 50 nm, as disclosed, for example, in the following Non-Patent Document 1, and that processability of a ceramics material also becomes better along with an increase in grain size. However, the technique disclosed in the Patent Document 3 has difficulty in increasing a grain size in a boron carbide sintered body, because boron carbide has relatively low sinterability. Moreover, the sintered body has extremely low processability, because boron carbide itself has a high hardness. This is a major factor causing an increase in cost in producing a boron carbide-based ceramics material. Therefore, there is a strong need for a boron carbide-based ceramics material having high rigidity but with excellent processability.